Stimulation of an immune response is dependent upon the presence of antigens recognized as foreign by the host immune system. Bacterial antigens such as Salmonella enterica and Mycobacterium bovis BCG remain in the phagosome and stimulate CD4 T-cells via antigen presentation through major histocompatibility class II molecules. In contrast, bacterial antigens such as Listeria monocytogenes exit the phagosome into the cytoplasm. The phagolysosomal escape of L. monocytogenes is a unique mechanism which facilitates major histocompatibility class I antigen presentation of listerial antigens. This escape is dependent upon the pore-forming sulfhydryl-activated cytolysin, listeriolysin O (LLO).
ActA is a surface-associated Listerial protein, and acts as a scaffold in infected host cells to facilitate the polymerization, assembly and activation of host actin polymers in order to propel the Listeria organism through the cytoplasm. Shortly after entry into the mammalian cell cytosol, L. monocytogenes induces the polymerization of host actin filaments and uses the force generated by actin polymerization to move, first intracellularly and then from cell to cell. A single bacterial protein, ActA is responsible for mediating actin nucleation and actin-based motility. The ActA protein provides multiple binding sites for host cytoskeletal components, thereby acting as a scaffold to assemble the cellular actin polymerization machinery. The NH2 terminus of ActA binds to monomeric actin and acts as a constitutively active nucleation promoting factor by stimulating the intrinsic actin nucleation activity. ActA and hly are both members of the 10-kb gene cluster regulated by the transcriptional activator PrfA, and is upregulated approximately 226-fold in the mammalian cytosol.
There exists a long-felt need to develop compositions and methods to enhance the immunogenicity of antigens, especially antigens useful in the prevention and treatment of tumors and intracellular pathogens.